Osmotic stress increases brain-derived neurotrophic factor messenger RNA expression in the hypothalamic supraoptic nucleus with differential regulation of its transcripts. Relation to arginine-vasopressin content.

We have shown that osmotic stress increases brain-derived neurotrophic factor (BDNF) mRNA in the supraoptic nucleus and that this increase seems to be determined by the high expression of transcripts containing exon I. The paraventricular nucleus is another hypothalamic neuronal subset where BDNF mRNA is also sensitive to osmotic stress stimulation. In this nucleus, transcripts containing exon I were not modified but only those containing exon II. By contrast, transcripts containing exon III did not exhibit any variation in our experimental conditions. The presence of BDNF mRNA in both paraventricular and supraoptic hypothalamic nuclei was recently reported. These nuclei are extremely sensitive to osmotic stimuli and their neurons secrete oxytocin and arginine-vasopressin in the posterior pituitary gland. This study was thus designed to investigate the possible involvement of BDNF in the response of supraoptic nucleus to osmotic stress stimulus. Osmotic stress was induced by hypertonic saline injection (1.35% NaCl) administered to animals 3 h before analysis. We used non-isotopic in situ hybridization to study the expression of BDNF mRNA and its transcripts with antisense riboprobes on histological brain sections, including paraventricular and supraoptic nuclei from control and osmotic stress-stimulated animals. To investigate a possible correlation between the expression of BDNF mRNA and arginine-vasopressin, the peptide content was analyzed by immunohistochemistry in both paraventricular and supraoptic nuclei at two different times after hyperosmotic injection. The results showed that BDNF mRNA expression preceded the arginine-vasopressin increase. In addition, on serial adjacent histological sections of supraoptic nucleus (10 microm), both BDNF and arginine-vasopressin mRNAs were visualized by isotopic in situ hybridization and the images were overlaid, showing that almost all of the hybridization signals were overlapped. Taken together our results are in keeping with the hypothesis that activation of the different BDNF promoters seems to be region-specific. Besides, the temporal correlation between both BDNF mRNA expression and arginine-vasopressin content, as well as the morphological vicinity between their respective producing cells in the supraoptic nucleus, suggest an autocrine or paracrine action for this neurotrophin in the regulation of arginine-vasopressin secretion.